The present invention relates to a process for the production of polymers of ethylene and an apparatus for operating the process.
Processes for polymerizing ethylene under high pressure (about 400 to 3000 bars) by means of either a free radical initiator, as in U.S. Pat. No. 3,255,171, or a catalytic system of the Ziegler type comprising an organo-metallic activator and a transition metal halide, as in British Pat. No. 1,441,115 have long been known.
In the continuous ethylene polymerization processes, the polymer produced is separated from the reaction mixture in a first separation zone under a pressure generally between 100 and 500 bars, and then in a second separation zone is maintained at a pressure as close as possible to atmospheric pressure: 2 atmospheres according to the examples in U.S. Pat. No. 3,551,397.
In the known polymerization processes cited above, the ethylene fraction separated in the second separation zone--a fairly large fraction since it generally represents 20 to 50% of the production of polymer--has to be passed to a booster to be compressed to the pressure of the ethylene feed flow, mixed with this flow, and compressed with it to the pressure of the first separation zone. The resulting mixture is then mixed with the ethylene fraction separated in the first separation zone and compressed with it to the working pressure of the reactor.
The process of recycling of ethylene cited above has two main disadvantages. Firstly it is wasteful of energy because it leads to the expansion followed immediately by recompression of a relatively large flow of ethylene (separated in the second separation zone), the supplementary cost in energy of this operation being all the greater because the pressure of the second zone of separation is lower. Secondly it permits only an incomplete degassing of the polymer produced, which results in difficulties in storing the polymers. The presence of residual gas in the storage areas causes a disagreeable odor, risks of explosion, and stress on the workers engaged in storage and handling. The development of standards in force in many countries in relation to toxicology, hygiene, and safety makes it likely that the present processes will soon no longer conform to these standards because they do not ensure adequate degassing of the polymers produced.
British Pat. No. 1,313,836 describes a process for the polymerization of ethylene under a pressure of 700 to 5000 bars at a temperature of 100.degree. to 400.degree. C., characterized by the presence of one or more high pressure separators, at least one of which works at a pressure of 500 to 1000 bars. According to the description in this patent, "high pressure" means at least 250 bars. This process, in a manner emphasized by the very high pressure chosen, has the same disadvantages as the processes cited above.